British Prime Minister David Cameron was holding talks Sunday with Russian President Vladimir Putin on Syria's civil war, a meeting which could set the tone for the G8 summit, with the West at odds with Moscow over the conflict.
Cameron is seeking to forge an international consensus on handling the unrest as he hosts the leaders of the world's top industrialised nations in Lough Erne, Northern Ireland, from Monday.
Washington has upped the ante on Syria by vowing to send military aid to rebel forces battling to topple President Bashar al-Assad after saying it had proof that his regime had crossed a "red line" by using chemical weapons on a small scale.
Russia, which has given the Syrian regime military support and ignored months of pleas from the West to rein in Assad, was dismissive of the US claims.
American officials will not reveal exactly what military support will go to the rebel Supreme Military Council, although by many estimates it will initially be assault rifles and ammunition.
Russian Foreign Minister Sergei Lavrov backed Assad on Saturday, saying there was no need for the Syrian leader to use chemical weapons against the rebels because his forces were making steady advances on the ground anyway.
"What sense is there for the regime to use chemical arms -- especially in such small amounts?" he asked.
The international community has long been divided over how to tackle the Syria conflict, which according to UN figures has cost at least 93,000 lives since March 2011.
Cameron had to meet Putin at the back entrance to Downing Street due to a large protest against the Turkish government blocking the main gates.
The president's plane arrived late at a military airbase in west London and was further held up by the protest.
Putin and Cameron shook hands and posed for photographs before heading inside.
Ahead of the talks, Cameron said it was essential to assist the moderate Syrian rebels prepared to work with the West before Al-Qaeda-linked extremists gained the upper hand in the opposition.
"I want to help the Syrian opposition to succeed and my argument is this: yes, there are elements of the Syrian opposition that are deeply unsavoury, that are very dangerous, very extremist, and I want nothing to do with them," he told Sky News television.
"But there are elements of the Syrian opposition who want to see a free, democratic, pluralistic Syria that respects the rights of minorities, including Christians, and we should be working with them."
Cameron has not said whether he favours sending weapons to the rebels, but he believes the lifting in May of a European Union arms embargo was essential to put pressure on Assad.
Putin will follow his talks with Cameron by meeting US President Barack Obama in Northern Ireland on Monday.
The fighting in Syria has taken on increasingly sectarian overtones, pitting a Sunni-led opposition against the Alawite-dominated regime.
US Secretary of State John Kerry, who is pushing for an international peace conference on Syria, warned on Saturday that the chances for a political settlement could be undermined by the regime's use of chemical weapons.
The G8 summit is also likely to consider the impact of the election of moderate cleric Hassan Rowhani as Iran's new president.
Putin urged him to forge closer ties with Moscow, while the United States offered the prospect of direct engagement with Iran but Israel called on world powers to keep up the pressure over the Islamic republic's nuclear drive.
Iran is a key ally of Syrian President Bashar al-Assad, staunchly backing his embattled government.
The Syrian opposition accuses Iran of providing Damascus with weapons and encouraging Lebanon's Shiite Hezbollah militia, which relies on Tehran for support, to dispatch fighters to Syria.
Syrian Prime Minister Wael al-Halqi said Damascus would seek to expand its relations further with Tehran following Rowhani's victory.
Pope Francis called on the G8 to push for "an immediate and lasting ceasefire" in Syria.
"Peace demands a far-sighted renunciation of certain claims, in order to build together a more equitable and just peace," he wrote to Cameron.
Besides Syria, Cameron wants the G8 summit to produce new agreements on tax, trade and financial transparency.